Method and apparatus for sound recording



Sept. 19, 1939. H L R v 2,173,222

v METHOD AND APPARATUS FOR SOUND RECORDING Filed April '30, 1936 2Sheets-Sheet l lhmcntor 9: (Ittorneg Sept. 19, 1939. H, LAR 2,173,222-

METHOD AND APPARATUS FOR SOUND RECORDING Filed April 30, 1936 2Sheets-Sheet 2 1 4 9 LP IE? 1 v I 2 101 w A J f Zmvcntor Patented Sept.19,1939

METHOD AND APPARATUS FOR SOUND RECORDING Herbert Belar, Philadelphia,Pa., assignor to Radio Corporation of America, a corporation of DelawareApplication April 30, 1936, Serial No. 77,168

Claims.

This invention relates to an apparatus for mixing sound from two or moresources and more particularly to a variety thereof wherein the soundfrom one source controls a characteristic 5 of the sound from the othersource or sources. In the recording of sound particularly for soundpicture records it is customary to provide a music record and then addto the music either at the time the picture is being taken or afterwardthe appropriate speech. This is usually done in rerecording, i. e. thepicture is taken and the voice is-recorded therewith. Thereafter, thedesired music is provided'in the form of a sound record and is runthrough a phonograph, preferably using a film music record and a filmphonograph while the voice record is similarly played back. the twobeing mixed and recorded as a single record. ,In doing this, it isnecessary that the voice be rendered loud and rather harsh if the musicbe kept at a substantially uniformlevel. As an alternative method, thelevel of the music has been decreased during the speech and increased toits normal value at the conclusion thereof. As pointed out in Patent No.1,888,467 to Mueller, this has been accomplished manually and the saidpatent proposes to do the same thing automatically, controlling themusic "directly by the voice impulses.

None of the foregoing methods are entirely satisfactory nor are they inaccord with ordinary orchestral practice.

Where voice s rendered with orchestral accompaniment it has not beenfound satisfactory in practice to reduce the volume of the entire :33orchestra during the vocal passages. On the contrary, usually only thoseinstruments are reduced in\volume or suppressed which, from their tonequality or pitch, would interfere with the audibility of the voice. 'Forexample, during vocal 4.1) pa a es. particularly of dance music, thedrums and other rhythm instruments are not suppressed or reduced involume but are kept at normal volume. On the other hand, with a sopranoor alto solo such instruments as the comet, the'clarinet 45 and sopranosaxophone may be either suppressed or reduced in volume, thecorresponding portion of the piano accompaniment may be either omittedor reduced in volume and instruments approaching the same frequencyrange, such as the 1) alto horn and the altosaxophones and cellos, maybe somewhat reduced in volume and caused to play a. harmonizing ratherthan a similar part.

On the other hand, instruments of much higher range and having such tonequality as not to 55 interfere with the voice rendition ,Such, forexample, as the violins and flute, may be maintained at full volume.

Similarly, in the rendition of a baritone or bass solo the conflictinginstruments such as the trombone, tenor saxophone, etc. may be sup- 6pressed while the instruments remote from the particular voice range areleft substantially unchanged.

It is the purpose of my invention to accomplish automatically in themixing of voice and 10 sound records what is accomplished in the mannerpointed out above in the original rendition of voice with properorchestral accompaniment, and I do this by decreasing the volume of themusic record in the particular tonal range 15 wherein it will conflictwith the voice record. It will be apparent that my novel method may beaccomplished manually, but I prefer, in order to more closely follow thevoice passages, to accomplish the control automaticallyfrom the voice 20passages themselves.

One object of my invention is to provide a novel method of making voiceand music records.

Another object of my invention is to provide a novel method ofsuppressing undesired portions 25 of music records during voicepassages.

Another object of my invention is to provide an apparatus for mixingvoice and music records and suppressing undesired portions of the musicrecord.

. Another object of my invention is to provide means for automaticallysuppressing undesired frequency ranges in the music record in accordancewith or by means of the voice input.

Other and incidental objects of my invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawings in which Figure 1 is adiagrammatic illustration of one arrangement of the apparatus,

Figure 2 is a schematic wiring diagram of the automatic frequencycontrol used in connection with Figure 1,

Figure 3 is a schematic wiring diagram of 9.45 second form of frequencycontrol which may be used in conjunction with Figure 1,

Figure 4-. is a schematic wiring diagram of a third circuit which I mayuse in conjunction with my invention as shown in Figure 1, and

Figure 5 is a diagrammatic illustration of a second arrangement ofapparatus which I may use in conjunction with any of the foregoingcircuits.

Referring, first, to Figure 1, the voice input which may be either a.microphone. a disc phonoiii graph, a film phonograph or any otherequivalent source of electrical waves corresponding to voice impulses isindicated at l. The corresponding source of music which may be anydesired variety and not necessarily the same type of source as source 8is indicated at 2. The frequencycontrol or frequency-selective amplifieris indi-- cated at t, the mixer at 4, the recording amplifier at 5 andthe recorder at The mixer includes manual control means TI and 8 fordeterminhng the relative volumes of the outputs from the sources I] and2 and a control means for deter= mining the total output from the twosources to the amplifier 5. The amplifier 5 may be of any appropriate orconvenient type having sufidcient voltage gain and power output toproperly actu= ate the recorder The recorder 65 is preferably,

as diagrammatically illustrated, a sound-=on-filmtrol amplifier 3 ofwhich one species is'shown in schematic form in Figure 2.

In the species of voice-controlled amplifier shown in Figure 2, thevoice input from the leads I0 is fed into the apparatus through thetransformer 2| and the music input from the source 2 is fed in throughthe transformer 22. In music recording, when there is no speech input,the

output from the transformer 22 is applied to the grid 23 of the tube 24in the manner shown, and the audio frequency output from the plate ofthe tube 24 passes through the reactor 25 and the capacitor 26 to thegrid of the tube 21 which further amplifies the impulses, passing themthrough the condenser 28 -to the output transformer 29. The tube 30 doesnot operate under these circumstances. When voice input is received fromthe leads II), it passes through the transformer 2[ to the amplifiertube 33 where it is amplified and passed through the condenser 34 to thetransformer 35. The output from the transformer 35 is fed through theleads 36 to the cathode heating circuit of the tube 30, the middle tapof the transformer being connected through the lead 31 to the groundlead 38 of the other portion of the amplifier. Approximate grid bias forthe tube 30 is provided by the C battery 3| and the potentiometer 32.The tube 30 .ls preferably of the variety known as a type 30 Radiotronwhich requires, for normal characteristics, a filament voltage of twovolts and a filament current of .060

ampere.

It will be apparent that when there is no voice input thereis no outputfrom the transformer 35 and therefore no emission from the filament ofthe tube 30, which remains at an infinite resistance. The reactor 25presents a high impedance to the high frequency components of the soundand is effective to produce between the anode and cathode of the tube 30a voltage which varies with frequency and increases with frequency.Accordingly, when input is applied across the leads ID the output fromthe transformer 35 serves to heat the filament of the tube 30 and tothereby present a relatively low resistance across the circuit. Onaccount of the characteristics of the inductance 25, the highfrequencies will be more 01 less y passed through the tube 30, therebyeliminating high frequencies from the output of the amplifier 3 inproportion to the amount of voice input received.

The amplifier shown in Figure 3 is of a slightly different type. In thisamplifier the input from the leads I0 is applied through a transformer2| to a tube 33 as before, the output from the tube 33 being appliedthrough a resistance-capacitance coupling to the tube 43. The input fromthe transformer 42 which is connected to the music source is appliedthrough a coupling arrangement hereinafter described to the tubes 44 and45. These tubes are preferably of the variable gain pentode type, suchfor example as the type 6D6 Radiotron and the type 58 triple-grid supercontrol Radiotron amplifier or 78 triple-grid super control Radiotronamplifier, for reasons which will hereinafter be explained. These tubeshave their cathodes connected to a common lead 46 with the usual by-passcondensers 4i and screen bias resistors 48 interposed, and derive theirplate and screen currents from the same battery 49 as the tube 33. Thelead 46 is connected to the 'midpoint of the transformer 42, of whichone side is connected to the tube 44 through the resistor 49 and thecondenser 50, while the other side is connected to the tube 45' throughthe resistor 5| and inductance 52. With the circuit shown and the tubesreferred to a negative grid bias is not always necessaryin the lead 46but if so desired bias may be supplied here by the insertion of anappropriate C battery or equivalent biasing circuit. The output from thetubes 44 and 45 is connected through the transformer 53 to the leads tothe control member I of the mixer 4.

It can be shown mathematically that if the square root of the inductanceof the choke 52 divided by the impedance of the condenser 50 1s equal tothe resistance of the resistors 49 or 5|, which are similar, then theamplification of the amplifier will be fiat over substantially theentire frequency range ifthe gain of the two amplifier tubes is thesame. If, however, the gain of these amplifier tubes 44 and 45 ischanged then the entire characteristic of the amplifier will be tiltedone way or the other about a predetermined frequency increasing the gainat high frequencies and decreasing the gain at low frequencies or viceversa, according to the direction of shift of the gain of the tubes. Thefrequency about which this tilt of the amplifier characteristic occursis determined by the relation of the condenser 50 and the.

inductance 52, i. c. this frequency equals the rc ciprocal of 21r timesthe square root of the product of the inductance and the capacitance,these units being expressed in farads, henries, ohms and cycles persecond. When the gain of the tube 45 is increased and that of the tube44 decreased a gain in high frequencies and decrease in low frequenciesresults. On the other hand, when the gain of the tube 44 is increasedand the gain on the tube 45 is decreased an increase in low frequenciesand decrease in high frequencies is through the battery 51 to the lead46. It will be apparent that when voice is applied through the circuitl0 current varying as the volume of the sound willpass through theresistors and 56 rendering the cathode end of the resistor 55 positiveand the'opposite end of the resistor 56 negative, thereby rendering theamplification-control grid of the tube 45 more negative and decreasingthe amplification thereof while at the same time correspondinglyincreasingthe amplification of the tube 44. It will thus be apparentthat any voice input will cause a decrease in high frequencies and ofincrease in low frequencies of the music transmitted. It will beapparent that this amplifier, as well as the form shown in Figures 2 and4, is not limited to use'in the arrangement shown in Figure '1 or Figure5. For example, the leads indicated at IU may be connected to the samesource of sound as the input of this amplifier indicated at 2, and inthis case the frequency response will be shifted according to theloudness of the input sound.

Further, instead of connecting my frequency control unit -3 to a sourceof audio input such as l, I may connect the leads ID to an appropriatesource of current such as a battery and a potentiometer, omitting theintervening tubes and transformers and I thereby have an amplifierhaving a remote frequency-response control.

In the third form of the circuit shown in Figure 4 the input from thesource 2 is divided, a portion pas-sing through the low-pass filterincluding the shunt condensers 60 and 6| and the series inductance 62 tothe transformer 63, and another portion passing through the high passfilter including the shunt inductances 64 and 55 and the seriescondenser 56 to the amplifier tube 61. The output from this tube 61passes through the transformer 68 to the transformer 63 and the outputfrom this transformer 63 passes to the mixer control 1, which is hereshown as a T pad arrangement of resistors. I

The tube 61 is of the triple-grid type including a control grid III, ascreen grid H and an amplification-control grid 12. The control grid andscreen grid are connected in the usual manner, the screen grid beingbypassed by the condenser 13. The input from the leads I0 is applied tothe grid-cathode circuit of the tube 14, the plate of which is suppliedthrough a resistor 15 from the B battery 16. The audio output of thistube is supplied through the condenser 11 to' the tube 18 which is atriode having the grid and plate connected together to act as a dioderectifier. The output from the tube 14 is rectified by the tube 18 andappears as a voltage drop across the resistor I9 and the condenser 80.This voltage drop is applied through the leads 81 across the cathode andamplification control grid of the-tube 61. In this type of tube as thegrid goes more negative the amplification decreases, and accordinglywhen the output from the tube 14 increases the amplification of .thetube 61 decreases thereby decreasing the amount of the higher audiofrequencies passing through the filter 64, 65, 66 and the transformer 68to the transformer 63.

The low and intermediate frequencies are, as shown in Figure 1,transmitted through a transformer 82 to the-mixer control 8 and from themixer control I and 8 to the volume control 9.

In the form of the invention shown inFigure 5 the input from the musicsource 2 is divided into three circuits passing through a high passfilter I00, a low pass filter HH and a band-pass filter I02. Theband-pass filter is so designed or adjusted asv to pass the frequencieswhich would interfere with the voice rendition which is to be used,while the high-pass and low-pass filters transmit respectivelyfrequencies higher than and lower than those which would causedifiiculty. The leads ii) in this case are con nected to a voicecontrolled amplifier 3 which is connected in the band passfilter'circuit only, and this control unit 3 may be made according toany of the foregoing circuits shown as applied to the high frequencyportion of the music. The current passed by the high-pass and low-passfilters and by the band-pass filter and the voice controlled amplifierare again combined andtransmitted to mixer at I. while the output fromthe source I is transmitted to the mixer at 8 and the combined output iscontrolled in volume by the control 9.

It will be apparent to those skilled in the art that my invention is notlimitedto the controlling of high frequencies or the controlling of aband of intermediate frequencies but that I may control bands offrequencies at either or both ends of the audio spectrum or at one oravoice channel, a thermionic tube connected from the connection of saidinductance and capacitance to the other side of said channel, and meansfor controlling the conductance of said thermionic tube in. accordancewith independent impulses whereby predetermined frequencies are bypassedby said tube in accordance with said other impulses.

2. A thermionic amplifier, including audio frequency current supplymeans, means for decreasing the amplification of high frequencyimpulses, means for simultaneously and similarly increasing theamplification of low frequency impulses, and means responsive to saidcurrent for controlling the aforesaid means.

3. A thermionic amplifier, including means for decreasing theamplification of high frequency impulses, means for simultaneously andsimilarly increasing the amplification of low frequency impulses, meansfor controlling the aforesaid means in accordance with otherhighfrequency impulses, and means for combining said impulses.

'4. A thermionic amplifier, including means for decreasing theamplification of high frequency impulses, means for simultaneously andsimilarly increasing the amplification of low frequency impulses, meansfor controlling the aforesaid means in accordance with other highfrequency impulses, and means for combining all of said impulses.

5. The method of mixing sounds comprising producing sounds having abroad frequency range, producing other sounds having a narrowerfrequency range, controlling the loudness of sounds corresponding tosaid narrower frequency range only in said sounds having a broadfrequency range in accordance with said sounds having a narrow frequencyrange, and combining said sounds.

HERBERT BELAR.

